1. Field of the Invention
The present invention relates to a magnetic detection apparatus for detecting the movement of an object by means of a magneto-electric conversion device which utilizes the current magnetic-effect of a magneto-resistive device, a Hall device or the like.
2. Description of Related Art
A conventional rotation detecting apparatus for detecting the movement of an object by using a magneto-electric conversion device is known by, for instance, JP-A 3-195970. In this apparatus, magneto-electric devices are provided in such a way that each of them faces gear teeth through an air gap. Variations in bias magnetic field generated by a bias magnet which variations are caused by the rotation of the gear and are converted by the magneto-electric device into an output electrical signal. The output electrical signal is then converted into binary data. By counting the number of pulses of the binary data or measuring the gap between two consecutive pulses or the pulse period, the rotational angle and the rotational speed can be found. As an apparatus of this type, a determination as to whether or not the rotation is in the opposite direction can also be made as is disclosed in JP-Y2 6-33419. In this apparatus, two component pairs each comprising a magneto-resistive device and a circuit for converting a signal output by the magneto-resistive device into binary data are provided. The binary data output by the circuit is used for determining whether the direction of the rotation is normal or reverse.
In several experiments conducted by the inventors, an attempt was made to detect a reference position by eliminating a tooth of the gear at the reference position. With a tooth eliminated, the gap between two consecutive pulses is changed and the change in pulse gap (or pulse period) can be observed as the reference position. However, the inventors were aware of the fact that the amplitude of an alternating current signal output by the magneto-resistive device on the boundary between a region with teeth arranged continuously and a location with a missing tooth for detecting the reference position inevitably increased. As a result, a problem was encountered when the alternating current signal output by the magneto-resistive device was amplified in preprocessing prior to the conversion process into binary data. On the boundary between the region with teeth arranged continuously and the location with a missing tooth, the pulse gap for detecting the reference position is found to be greater than the pulse gap (or the pulse period) in the region with teeth arranged continuously after the binary conversion, making it impossible to pass on accurate rotation angle information after the binary conversion. In order to determine whether or not the rotation is in the reverse direction, it is necessary to provide two component pairs each comprising a magneto-resistive device and a circuit for converting a signal output by the magneto-resistive device into binary data as described above, making the configuration of the circuit complex.
In the detection of the rotation of an object of observation by means of a magnetic pickup, the output voltage V is equal to d.PHI./dt. As a result, the magnitude of the output voltage is small at a low rotational speed, making it difficult to put the apparatus to practical use.
A magneto-resistive device serving as a sensor provides an output with a constant magnitude without regard to the rotational speed. For this reason, the magneto-resistive device offers a benefit in that it can also be used for low rotational speeds as well. On the other hand, since the magnitude of the output is small, it must be amplified.
If the amplitude of a signal output by an amplifier for amplifying the signal output by the magneto-resistive device is increased excessively, however, too large a voltage is applied to devices employed in the circuit, giving rise to a possibility of malfunction. For this reason, in general, the signal output by the amplifier is limited so that its magnitude does not exceed a predetermined value. An amplification factor must thus be set so that, on the boundary between the region with teeth arranged continuously and the region with a missing tooth, the amplitude of the amplified signal for detecting the reference position is smaller than a predetermined limit. As a result, the amplitude of the amplified signal in the region with teeth arranged continuously can not be amplified sufficiently, giving rise to a problem that it is difficult to distinguish the signal output by the magneto-resistive device from existing noise.